Typically, a disk drive includes a stack of several data storage disks having concentric tracks capable of storing data. A number of read/write transducers, located on actuator arms, are used to communicate with each data storage disk. The disks are spun at a high rotational speed causing the transducers to float above the disks on a small cushion of air.
One concern with disk drive operation is the potentially detrimental effects on a read or write operation when the disk drive, or a computing unit in which the disk drive resides, is subjected to a physical shock. If the shock is large enough, transducer movement could be induced that causes information to be read from or written to the wrong track. As disk drives continue to become smaller, with reduced tolerances and higher information storage densities, the risk of significant problems from a physical shock increases. Also, as computers continue to become more portable, they will continue to be used in ever more severe environments, which will require more reliable disk drive operation under a variety of operating conditions.
One method proposed for addressing problems cause by physical shocks is to modify the mechanical structure of the disk drive to absorb the shock. For example, U.S. Pat. No. 5,004,207 discloses the use of shock absorbers on the disk drive support structure to dampen shocks or vibrations. Such a shock dampening approach may provide some relief for small physical shocks or vibrations, but is not adequate for the large physical shocks frequently encountered by portable computers, such as laptops.
Another proposed approach for addressing problems caused by physical shocks is through use of off-track detectors. For example, U.S. Pat. No. 5,126,859 discloses a disk drive in which a detector constantly monitors transducer movement. During normal operation, the transducer is positioned between predetermined limits on the track. If the transducer moves off a predetermined track, the read or write operation is suspended and data is either re-written or re-read after the transducer has been re-positioned to the correct track. The effectiveness of off-track detectors is limited by the often significant time required for the system to respond to a physical shock. The response time is often not sufficiently fast to protect a read or write error, especially for relatively large shocks.
Another proposed approach to addressing problems caused by physical shocks, is to use a shock sensing device. For example, U.S. Pat. No. 5,491,394 discloses the use of an EMF coil that generates a voltage signal when a magnet, responding to a physical shock, is moved across the coil. Also, U.S. Pat. No. 5,227,929 discloses use of an accelerometer to detect a physical shock, such as when the disk drive is dropped or otherwise moved. Both these shock detection units typically produce an output signal that is compared to a threshold value. When the output signal from the sensor is greater than the threshold, the operation of the disk drive is halted.
One problem with these shock detecting systems is that the circuitry often contains significant background noise that can interfere with proper comparison of the output signal with the threshold. This problem is even more acute for computers, such as laptops, used in portable applications. This problem occurs because background signals in the circuit can vary considerably with changes in ambient operating conditions, and especially with changes in temperature. In some instances, the background noise may become so large that the noise distorts the output signal to such a large degree that the comparison of the output signal with the threshold value becomes unreliable as an accurate indicator of whether a shock is of significant size to warrant interruption of a read or write operation.
There is a significant need for improved methods for addressing operating problems that can occur in disk drives due to physical shocks as computers continue to be subjected to more severe and highly variable ambient operating conditions. This need is intensified as computers become smaller and more portable. More particularly, this need is intensified as information storage densities and track densities within disk drives increase causing the tracks to be spaced closer together. As such, it becomes critically important to accurately control movement of the transducer relative to the tracks.